Embodiments of a key
system for filters and their connecting heads, brackets, or other holders are shown. The filter
cartridge and its holder each have a keyed surface, one being a protruding “key” and one being a recessed “lock”. Cooperation of these keyed surfaces is required in order for the filter
cartridge to be installed in the holder, so that mis-matched cartridges cannot be installed into the holder, for example, to prevent a particular type of
cartridge from being placed in a
filtration or other process where it would be inappropriate, or undesired. The keyed surfaces are selectively locate-able preferably at different circumferential locations on a perimeter of the filter and a corresponding location on a perimeter of the head / holder. The perimeter may be, for example, on an outer
shoulder surface of a filter and an inner surface of a valve head, or on outer and inner surfaces of connectors that provide a liquid seal between the filter and the head / holder.